This invention relates to a process for preparing a homogeneous, particulate polyphosphate mixture from an aqueous solution of mono- and di- alkali metal orthophosphates.
Sodium polyphosphates, such as sodium pyrophosphates, sodium tripolyphosphate and sodium metaphosphate, are produced commercially from an aqueous solution of monosodium phosphate, and disodium phosphate or mixtures thereof. This "wet mix" is typically spray-dried to produce granules of the dehydrated orthophosphates, which are subsequently converted to polyphosphates by calcining the particles at elevated temperatures, e.g. 350.degree.-550.degree. C. The wet mix itself is typically prepared by reacting sodium carbonate and phosphoric acid in an aqueous medium.
The composition of the final polyphosphate product will depend on the sodium to phosphorus mole ratio of the wet mix. For instance, a sodium to phosphorus mole ratio of 6:3 results in the production of sodium pyrophosphate as follows: EQU 2Na.sub.2 HPO.sub.4 .fwdarw.Na.sub.4 P.sub.2 O.sub.7 +H.sub.2 O
Sodium tripolyphosphate is produced with a sodium to phosphorus mole ratio of 5:3 as follows: EQU 2Na.sub.2 HPO.sub.4 +NaH.sub.2 PO.sub.4 .fwdarw.Na.sub.5 P.sub.3 O.sub.10 +2H.sub.2 O
Each polyphosphate is commercially produced as a relatively pure composition containing less than about 10% of impurities such as orthophosphates and other polyphosphates.
Occasionally, a customer will desire a mixture of two different polyphosphates, such as sodium pyrophosphate and sodium tripolyphosphate, or a mixture of different alkali metal salts of one or more polyphosphates, such as sodium and potassium pyrophosphate or a mixture of sodium and potassium tripolyphosphate and pyrophosphate. Such mixtures are typically prepared by physically blending the individual compounds. However, there are several disadvantages associated with this approach. The blending or mixing must be complete in order to insure a homogeneous mixture when the blended material is packaged in small containers. Moreover, the blending of granular compounds such as these requires close attention to particle size distributions and particle densities in order to prevent segregation of the particles upon shipment in containers or in bulk quantities. Finally, intensive blending tends to cause a breakdown of granular particles to smaller particle sizes or powders which may be undesirable.
The preparation of intimate granular mixtures of sodium tripolyphosphates and sodium sulfate is disclosed in the prior art. Both U.S. Pat. No. 3,397,948, issued Aug. 20, 1968 to Mesmer, and U.S. Pat. No. 3,684,724, issued Aug. 15, 1972 to Dyer et al, disclose mixtures of sodium sulfate and sodium tripolyphosphates. The mixtures are prepared as granular compositions from aqueous solutions of monosodium and disodium orthophosphate salts and sodium sulfate. Sodium sulfate is typically used as an extender for sodium tripolyphosphate in detergent applications.
It is, therefore, a principle object of the present invention to prepare homogeneous mixtures of alkali metal polyphosphates which are superior to the physical blends which are currently available.